The most well known and widely utilized electrostatic printing technique is that of xerography wherein latent electrostatic images formed on a charge-retentive surface, such as a roller, are developed by the use of a suitable toner material to render the images visible, the images being subsequently transferred to an information carrier.
This process is called an indirect process because it first creates a visible image on an intermediate surface and then transfers that image to an information carrier.
Another method of electrostatic printing is one that has come to be known as direct electrostatic printing. This method differs from the aforementioned xerographic method in that charged pigment particles (toner) are deposited directly onto an information carrier to form a visible image. In general, this method includes the use of electrostatic fields controlled by addressable electrodes for allowing passage of toner particles through selected apertures in a printhead structure. A separate electrostatic field is provided to attract the toner particles to an information carrier in an image configuration.
The distinguishing feature of direct electrostatic printing is its simplicity of simultaneous field imaging and particle transport to produce a visible image on the information carrier directly from computer generated signals, without the need for those signals to be intermediately converted to another form of energy such as light energy, as is required in electrophotographic printers, e.g. laser printers.
U.S. Pat. No. 5,036,341, granted to Larson, discloses a direct printing device and a method to produce text and pictures on an image receiving substrate directly from computer generated signals. According to that method, a control electrode array, formed of a latticework of individually controlled wires, is positioned between a back electrode and a rotating particle carrier. An image-receiving substrate, such as paper, is then positioned between the back electrode and the control electrode array.
A uniform electric field is generated between a high potential on the back electrode and a low potential on the particle carrier thereby to attract the toner particles from the surface of the particle carrier and to create a particle stream toward the back electrode. The particle stream is modulated by a series of voltage sources which apply an electric potential to selected individual wires of the control electrode array to produce electrostatic fields which permit or restrict particle transport from the particle carrier. In effect, these electric fields "open" or "close" selected apertures in the control electrode array to the passage of toner particles by influencing the attractive force from the back electrode. The modulated stream of charged particles allowed to pass through selected apertures impinge upon a print-receiving medium interposed in the particle stream to provide line-by-line scan printing to form a visible image.
The control electrode array described in the above mentioned patent is in the form of a lattice of individual wires arranged in rows and columns. A control electrode array operating according to the described principle may, however, have any one of several other designs. Generally, the array is a thin sheet-like element, referred to as a Flexible Printed Circuit or FPC, comprising a plurality of addressable control electrodes and corresponding voltage signal sources connected thereto for attracting charged toner particles from the surface of a particle carrier to an information carrier. A sequence of electronic signals, defining the image information, is converted into a pattern of electrostatic fields which locally modify the uniform field from a back electrode, thereby selectively permitting or restricting the transport of charged particles from the particle carrier and producing an image pattern corresponding to the electrostatic field pattern onto the information carrier.
A flexible control array or FPC as disclosed in, for example, U.S. Pat. No. 5,121,144, also granted to Larson, is made of a flexible, electrically insulating, non-rigid material, such as polyimide or the like, which is provided with a multitude of apertures and is overlaid with a printed circuitry whereby the apertures in the material are arranged in rows and columns and are surrounded by ring-shaped electrodes. A uniform electrostatic field generated by a back electrode attracts toner particles from a particle source to create a particle stream through the FPC toward the back electrode. All control electrodes are initially at a white potential, V.sub.w which means that toner transport from the particle carrier toward the back electrode is inhibited. As image locations on an information carrier pass beneath the apertures, selected control electrodes are set to a black potential V.sub.b to produce an electrostatic field drawing the toner particles from the particle source. The charged toner particles pass through the apertures in the FPC and are subsequently deposited on the information carrier in the configuration of the desired image pattern. The toner particle image is then made permanent by using heat and pressure to fuse the toner particles to the surface of the information carrier.
The variable electric forces applied to the individual control electrodes act either attracting or repelling on the toner particles positioned on the surface of the particle carrier. The electric forces must be carefully regulated to be above or below a predetermined transport threshold value corresponding to a print mode and a no print mode respectively. The threshold value is strongly dependent on the gap distance between the FPC and the surface of the particle carrier. The gap distance is generally in the order of about 50 microns and may vary within .+-.5 microns without severely affecting the print quality. Therefore, it is essential to provide a constant and uniform gap distance to maintain high print quality.
Further, when a control electrode is in the print mode, the attractive force must not be changed until the toner particles have gained sufficient momentum to pass through the corresponding aperture in the FPC. The time required for particles to be transported through an aperture must also be accurately controlled as a function of the gap distance between the FPC and the surface of the particle carrier.
Accordingly, even very minor variations in the gap distance between the FPC surface and the particle carrier surface may significantly and adversely affect the accuracy of the print control function, resulting in undesired size variation or density variation of the printed dots and degradation of the print readability.
It is desirable to arrange the FPC as closely to the toner carrier as possible without contacting the toner layer. Since the gap distance, as mentioned, typically is in the order of about 50 microns, even the slightest mechanical imperfections may cause a drastic degradation of the print quality.
A frequently used type of particle carrier is in the form a smooth cylindrical sleeve. However, in reality such a sleeve is never either perfectly cylindrical or perfectly smooth. In addition to the defects that may be found in the cylindrical sleeve, the layer of toner particles coating the sleeve may have a thickness which is slightly non-uniform. Further, the diameter of the particles themselves may vary and their shape may show deviations from an ideal spherical shape. These examples illustrate only a few of the numerous irregularities which may cause variations in the actual gap distance found between the FPC and the particle carrier.
A further source of variations in the gap distance is the mounting of the FPC in the print head structure. During the positioning of the FPC in alignment with the particle carrier, the FPC material may be deformed into a slight wave-shape due to non-uniform tension being applied to the material. Likewise, forces arising from the printing process itself may cause deformation which will affect the gap distance. Accordingly, to achieve a minimal and constant gap distance between the FPC and the particle carrier, while at the same time producing a uniform tension over the whole FPC surface and further to maintain these conditions during the whole print procedure, has proven to be one of the most critical steps of a direct printing method.
The positioning step is particularly important in order to achieve an improved print quality by enhancing the grey scale capability of the print head. The image configuration is formed by dots having variable form and/or darkness to create different shades in the range between white and maximal darkness. The control signals can be modulated with high precision to allow a desired amount of toner particles to be transported through each aperture in the FPC, the amount of toner particles transported through each aperture thereby corresponding to a specific grey level or shade. To obtain a satisfying grey scale capability when using a direct printing method, it is thus highly desirable to eliminate or at least considerably reduce the problems associated with irregularities occurring in the gap distance between the FPC and the particle carrier.
Therefore, to ensure a uniform print quality and enhance the grey scale capability of a direct electrostatic printing process, the need has been identified for an improved print head structure offering the required surface evenness, alignment and tension uniformity to the FPC material used in the process.